"Mozilla today announced Boot to Gecko , a very ambitious project that aims to create a 'complete, standalone operating system for the open web'. This project's goal is to develop what seems like a ChromeOS-like operating system where all the apps are based on HTML5."

The idea is that an HTML5 app is very portable, and the barrier to entry is low. All you really need to get started is a web browser and a text editor. Contrast that to iOS where you need OS X, Xcode, and a developers license; or Android where you need the ADK and generally a real device (the emulator is not very good).

It's also not very hard to create a quick native wrapper for your HTML5 app, and several solutions already exist (eg. Phonegap) that expose native APIs via Javascript.

Of course you're still at the mercy of whatever parts of HTML5 the mobile browsers support, which I think is one of Mozilla's really valid points here.